The present invention relates generally to a system for assisting the heart and, in particular, to an extracardiac pumping system and a method for supplementing the circulation of blood through the patient using a minimally invasive procedure.
During the last decade, congestive heart failure (CHF) has burgeoned into the most important public health problem in cardiovascular medicine. As reported in Gilum, R. F., Epidemiology of Heart Failure in the U.S., 126 Am. Heart J. 1042 (1993), four hundred thousand (400,000) new cases of CHF are diagnosed in the United States annually. The disorder is said to affect nearly 5 million people in this country and close to 20 million people worldwide. The number of hospitalizations for CHF has increased more than three fold in the last 15 years. Unfortunately, nearly 250,000 patients die of heart failure annually. According to the Framingham Heart Study, the 5-year mortality rate for patients with congestive heart failure was 75 per cent in men and 62 per cent in women (Ho, K. K. L., Anderson, K. M., Kannel, W. B., et al., Survival After the Onset of Congestive Heart Failure in Framingham Heart Study Subject, 88 Circulation 107 (1993)). This disorder represents the most common discharge diagnosis for patients over 65 years of age. Although the incidence of most cardiovascular disorders has decreased over the past 10 to 20 years, the incidence and prevalence of congestive heart failure has increased at a dramatic rate. This number will increase as patients who would normally die of an acute myocardial infarction (heart attack) survive, and as the population ages.
CHF manifests itself primarily by exertional dyspnea (difficult or labored breathing) and fatigue. Three paradigms are used to describe the causes and therapy of CHF. The first views this condition in terms of altered pump function and abnormal circulatory dynamics. Other models describe it largely in terms of altered myocardial cellular performance or of altered gene expression in the cells of the atrophied heart. In its broadest sense, CHF can be defined as the inability of the heart to pump blood throughout the body at the rate needed to maintain adequate blood flow, and many of the normal functions of the body.
To address CHF, many types of cardiac assist devices have been developed. A cardiac or circulatory assist device is one that aids the failing heart by increasing its pumping function or by allowing it a certain amount of rest to recover its pumping function. Because congestive heart failure may be chronic or acute, different categories of heart assist devices exist. Short of a heart transplant, at least two types of chronic heart assist systems have been developed. One type employs a full or partial prosthetic connected between the heart and the aorta, one example of which is commonly referred to as a LVADxe2x80x94Left Ventricular Assist Device. With reference to FIG. 1 herein, one example of a LVAD 2 is shown. The LVAD comprises a pump and associated valves 4 that draws blood directly from the apex of the left ventricle 6 and directs the blood to the aortic arch 8, bypassing the aortic valve. In this application, the left ventricle stops functioning and does not contract or expand. The left ventricle becomes, in effect, an extension of the left atrium, with the LVAD 2 taking over for the left ventricle. The ventricle, thus, becomes a low-pressure chamber. Because the intent is to take over for the left ventricle, the LVAD operates by pumping blood at cardiac rates. With an LVAD, oxygenated blood circulation is established sufficient to satisfy the demand of the patient""s organs.
Another type of chronic heart assist system is shown in U.S. Pat. No. 5,267,940 to Moulder. Moulder describes a pump implanted into the proximal descending aorta to assist in the circulation of blood through the aorta.
In addressing acute CHF, two types of heart assist devices have been used. One is counterpulsatory in nature and is exemplified by an intra-aortic balloon pump (IABP). With an IABP, the balloon is collapsed during isovolumic contraction, providing a reduced pressure against which the heart must pump blood, thereby reducing the load on the heart during systole. The balloon is then expanded, forcing blood omnidirectionally through the arterial system. Another example of this first type employs one or more collapsible chambers in which blood flows passively into the chamber during systole, as is shown in U.S. Pat. No. 4,240,409 to Robinson et al. The chamber is then collapsed and the blood forcibly returned to the aorta. These devices simulate a chamber of the heart and depend upon an inflatable bladder to effectuate pumping action, requiring an external pneumatic driver.
A second type of acute assist device utilizes an extracorporeal pump, such as the Biomedicus centrifugal pump, to direct blood through the patient while surgery is performed on the heart. In one example, described in U.S. Pat. No. 4,968,293 to Nelson, the heart assist system employs a centrifugal pump in which the muscle of the patient is mounted to add pulsatility to the blood flow. The Nelson device is used to bypass a portion of the descending aorta.
Another device, shown in U.S. Pat. No. 4,080,958 to Bregrnan et al., utilizes an inflatable and collapsible bladder to assist in blood perfusion during heart trauma and is intended to supplement a conventional heart-lung machine by imparting pulsatile actuation. In the primary embodiment disclosed in Bregman, the balloon is controlled to maintain sufficient pressure at the aortic root during diastole to ensure sufficient blood perfusion to the coronary arteries. In an alternative embodiment, a low resistance outlet from the aorta to the inferior vena cava is provided to reduce the aortic pressure during systole, thus, reducing the hemodynamic load on the left ventricle.
Other devices, such as that shown in U.S. Pat. No. 4,034,742 to Thoma, depend upon interaction and coordination with a mechanical pumping chamber containing a movable pumping diaphragm. These devices are intended primarily for application proximate the heart and within the patient""s thorax, requiring major invasive surgery.
Many CHF devices are acutely used in the perioperative period. For example, U.S. Pat. No. 4,995,857 to Arnold discloses a perioperative device to pump blood at essentially cardiac rates during surgery when the heart has failed or has been stopped to perform cardiac surgery. The Arnold system temporarily replaces the patient""s heart and lung temporarily and pumps blood at cardiac rates, typically 5 to 6 liters/min. Like all systems that bypass the heart and the lungs, an oxygenator is required. Of course, with any system that includes an oxygenator, such as the conventional heart-lung machine, the patient cannot be ambulatory.
With early IABP devices, a polyurethane balloon was mounted on a vascular catheter, inserted into the femoral artery, and positioned in the descending aorta just distal to the left subclavian artery. The balloon catheter was connected to a pump console that pumped helium or carbon dioxide into the balloon during diastole to inflate it. During isovolumic contraction, i.e., during the brief time that the aortic valve is closed and the left ventricle continues to contract, the gas used to actuate the balloon was rapidly withdrawn to deflate the balloon. This reduced the pressure at the aortic root when the aortic valve opened. In contrast, during diastole, the balloon was inflated, causing the diastolic pressure to rise and pushing the blood in the aorta distally towards the lower part of the body (on one side of the balloon) and proximally toward the heart and into the coronary arteries (on the other).
The major advantage in such a counterpulsation device was systolic deflation, which lowered the intra-aortic volume and pressure, reducing both afterload and myocardial oxygen consumption. In other words, when the balloon is inflated, it creates an artificially higher pressure in the aorta, which has the ancillary benefit of greater perfusion through the coronary arteries. When the balloon deflates, just before the aortic valve opens, the pressure and volume of the aorta decrease, relieving some of the hemodynamic burden on the heart. These physiologic responses improved the patient""s cardiac output and coronary circulation, temporarily improving hemodynamics. In general, counterpulsation with an IABP can augment cardiac output by about 15%, this being frequently sufficient to stabilize the patient""s hemodynamic status, which might otherwise rapidly deteriorate. When there is evidence of more efficient pumping ability by the heart, and the patient has moved to an improved class of hemodynamic status, counterpulsation can be discontinued, by slowly weaning while monitoring for deterioration.
Until 1979, all IABP catheters were inserted via surgical cutdown, generally of the femoral artery. Since then, the development of a percutaneous IABP catheter has allowed quicker, and perhaps safer, insertion and has resulted in more expeditious institution of therapy and expansion of clinical applications. Inflation and deflation of the balloon, however, requires a pneumatic pump that is sufficiently large that it must be employed extracorporeally, thereby restricting the patient""s movements and ability to carry out normal, daily activities. IABP devices are, thus, limited to short term use, on the order of a few days to a few weeks.
As discussed above, a variety of ventricular assist pumping mechanisms have been designed. Typically associated with LVADs are valves that are used in the inlet and outlet conduits to insure unidirectional blood flow. Given the close proximity of the heart, unidirectional flow was necessary to avoid inadvertent backflow into the heart. The use of such valves also minimized the thrombogenic potential of the LVAD device.
Typically, the pump associated with older LVADs was a bulky pulsatile flow pump, of the pusher plate or diaphragm style, such as those manufactured by Baxtor Novacor or TCI, respectively. Given that the pump was implanted within the chest and/or abdominal cavity, major invasive surgery was required. The pumps were typically driven through a percutaneous driveline by a portable external console that monitors and reprograms functions.
Alternatively, rotary pumps, such as centrifugal or axial pumps, have been used in heart assist systems. With centrifugal pumps, the blood enters and exits the pump practically in the same plane. An axial pump, in contrast, directs the blood along the axis of rotation of the rotor. Inspired by the Archimedes screw, one design of an axial pump has been miniaturized to about the size of a pencil eraser, although other designs are larger. Despite its small size, an axial pump may be sufficiently powerful to produce flows that approach those used with older LVADs. Even with miniaturized pumps, however, the pump is typically introduced into the left ventricle through the aortic valve or through the apex of the heart, and its function must be controlled from a console outside the body through percutaneous lines.
All of these heart assist systems referred to above serve one or both of two objectives: (1) to improve the performance of a patient""s operative-but-diseased heart from the minimum, classified as NYHAC Class IV, to practically normal, classified as I or 0; or (2) to supplement oxygenated blood circulation through the patient to satisfy organ demand when the patient""s heart is suffering from CHF. With such systems, extreme pumping and large amounts of energy, volume, and heat dissipation are required.
Many of these heart assist systems have several general features in common: 1) the devices are cardiac in nature; i.e., they are placed directly within or adjacent to the heart, or within one of the primary vessels associated with the heart (aorta), and are often attached to the heart and/or aorta; 2) the devices attempt to reproduce pulsatile blood flow naturally found in the mammillary circulatory system and, therefore, require valves to prevent backflow; 3) the devices are driven from external consoles, often triggered by the electrocardiogram of the patient; and 4) the size of the blood pump, including its associated connectors and accessories, is generally unmanageable within the anatomy and physiology of the recipient. Due to having one or more of these features, the prior art heart assist devices are limited in their effectiveness and/or practicality.
Many of the above identified prior art systems, generally referred to as Mechanical Circulatory Assist Devices, are not the only means, however, used to treat patients with congestive heart failure (CHF). Most CHF patients are prescribed as many as five to seven different drugs to ameliorate their signs and symptoms. These drugs may include diuretics, angiotensin converting enzyme (ACE) inhibitors, beta-blockers, cardiac glycosides, and peripheral vasodilators. The rationale for pharmacological intervention in heart failure include minimizing the load on the heart, improving the pumping action of the heart by enhancing the contractility of the muscle fibers, and suppression of harmful neurohormonal compensatory mechanisms that are activated because of the decreased pumping function of the heart.
Noncompliance with what is often a complex drug regime may dramatically adversely affect the recovery of a CHF patient leading to the need for hospitalization and possibly morbidity and mortality. In addition, ACE inhibitors and diurectics can cause hypotension, which leads to decreased organ perfusion or an increasing demand on the heart to pump more blood. This leads to an inability, in many cases, to prescribe the most effective dosage of ACE inhibitors and a less than optimum outcome for the patient. Patients suffering from CHF with the underlying cause of mitral valve insufficiency have been able to have their diuretics reduced following surgical repair of their mitral valve. This is due to an increased cardiac output and arterial pressures (as a result of the correction of the problem) resulting in more effective organ perfusion. With the reduction in the use of diuretics and the resultant hypotension, more effective dosages of ACE inhibitors can be used with more favorable outcomes. In addition, it is easier for the patient to follow a less complex drug regime, eliminating the costly and life threatening risks associated with noncompliance.
When blood flow through the coronary arteries falls below the level needed to provide the energy necessary to maintain myocardial function, due often to a blockage in the coronary arteries, a myocardial infarction or heart attack occurs. This is a result of the blockage in the coronary arteries preventing blood from delivering oxygen to tissues downstream of the blockage. The closer the blockage is to the coronary ostia, however, the more severe and life threatening the myocardial infarction. The further the location of the blockage is from the coronary ostia, the smaller the area of tissue or myocardium that is at risk. As the energy stored in the affected area decreases, myocardial cells begin to die. The larger the area that dies due to the loss of oxygen, the more devastating the infarction. To reduce the area at risk, at least two known options are to either increase the oxygen supply to the affected area or decrease the energy demands of the heart to prolong energy stores until the blockage can be removed or reduced. One particular method to increase blood flow, thereby increasing delivery of oxygen to the affected area, is through a technique called retroperfusion. This is accomplished by passing a cannula into either the right or left ventricle (depending on the area of the blockage) and perfusing oxygenated blood retrograde up the coronary artery on the downstream side of the blockage. Another method is to use drugs to increase the force of contraction of the myocardium, creating increased blood flow across the blocked area. Yet another method is to use drugs, such as aspirin or TPA, to reduce the viscosity (thin out) of the blood, thus, allowing more blood to pass by the blockage. The goal of all of these methods is to increase the delivery of oxygen to the tissue at risk. The alternative option mentioned above is to reduce the energy demands of the myocardium and increase the amount of time before irreversible damage occurs. This can be accomplished by reducing the workload of the left ventricle (which is the largest energy-consuming portion of the heart). An IABP is placed into the aorta and used as described above, resulting in a decreased afterload on the heart and increased perfusion of the coronary arteries and peripheral organs. An alternative way to reduce myocardial oxygen demand is to reduce the volume of blood the left ventricle must pump. This can be accomplished by reducing the load on the left ventricle, such as in a cardiopulmonary bypass or use of an LVAD. Unloading the left ventricle decreases the energy requirements of the myocardium and increases the amount of time before irreversible damage occurs. This provides an opportunity to more effectively remove or decrease the blockage and salvage myocardial function. To be successful, each of these techniques must be implemented within a short amount of time after the onset of a myocardial infarction. The disadvantage, however, is that each of these techniques can only be performed in an emergency room or hospital setting. Unless the patient is already in the hospital when the myocardial infarction occurs, there is usually some level of irreversible damage and subsequent loss of myocardial function.
It would be advantageous, therefore, to employ a heart assist system that avoids major invasive surgery and also avoids the use of peripheral equipment that severely restricts a patient""s movement. It would also be advantageous to have such a heart assist system that can be employed in a non-hospital setting for ease of treating acute heart problems under emergency conditions.
The object of the present invention is to address the aspect of CHF that results from altered pump function and abnormal circulatory dynamics while overcoming the limitations of prior art heart assist systems. Without functioning as a bypass to one or more of a patient""s organs, the present invention comprises an extracardiac pumping system for supplementing the circulation of blood through the patient without any component thereof being connected to the patient""s heart or primary vessel. Thus, it is extracardiac in nature. With the ability to be applied within a minimally invasive procedure, the present invention significantly improves the condition of the patient suffering from CHF, resulting in the patient feeling much better, even where CHF continues. An ancillary but important benefit of the present invention is the ability to apply the present invention in such a way as to also reduce the pumping load on the heart, thereby potentially permitting the heart to recover during use. With the present invention, no bulky pump, valves or oxygenator are required, and no thoracic invasion with major cardiac surgery is required. Indeed, a significant advantage of the present invention is its simplicity while achieving extraordinary results in improving the condition of a patient suffering from CHF.
The extracardiac system of the present invention preferably comprises, in one example, a rotary pump configured to pump blood through the patient at subcardiac rates; i.e., at a flow rate significantly below that of the patient""s heart. Other types of pumps or flow generating mechanisms may be effective as well. Pumping the blood tends to revitalize the blood to a certain extent by imparting kinetic and potential energy to the blood discharged from the pump. Importantly, the preferred pump for the present invention pumping system is one that requires a relatively low amount of energy input, when compared to prior art pumps designed to pump at cardiac rates. The pump may be implanted or not, depending upon the capability, practicality, or need of the patient to be ambulatory.
The present system also comprises an inflow conduit fluidly coupled to the pump, to direct blood to the pump from a first peripheral blood vessel, and an outflow conduit fluidly coupled to the pump, to direct blood from the pump to a second peripheral blood vessel. The connection of the inflow and outflow conduits to the blood vessels is performed subcutaneously; not so deep as to involve major invasive surgery. In other words, minimally subdermal. This permits application of the connections in a minimally-invasive procedure. Preferably, the connections to the blood vessels are just below the skin or just below the first layer of muscle, depending upon the blood vessels at issue or the location of the connection, although slightly deeper penetrations may be necessary for some patients.
In one embodiment of the extracardiac system, the pump is a continuous flow pump, a pulsatile pump, and/or a pump that is configured to generate flow in both a continuous and pulsatile format. The pump may be implantable and is used to connect two peripheral arteries, such as the femoral artery at the inflow and the left axillary artery at the outflow, although other peripheral blood vessels are contemplated, including other arteries and/or veins, as well as any singular and/or cumulative combination thereof. An alternative embodiment employs both a continuous flow and a pulsatile flow pump connected in parallel or in series and operating simultaneously or in an alternating fashion. Yet another alternative embodiment employs a rotary pump that is controllable in a synchronous copulsating or counterpulsating fashion, or in some out-of-phase intermediate thereof. In one application, it is contemplated that the present invention be applied such that the heart experiences a reduced pressure at the aortic root during systole (afterload) and/or a reduced left ventricular end diastolic pressure (pre-load), thus reducing the hemodynamic burden or workload on the heart and, thus, permitting the heart to recover.
It is contemplated that, where the entire system of the present invention is implanted, that it be implanted subcutaneously without the need for major invasive surgery and, preferably, extrathoracically. For example, the pump may be implanted in the groin area, with the inflow conduit attached to the femoral or iliac artery proximate thereto and the outflow conduit attached to the axillary artery proximate the shoulder. It is contemplated that the outflow conduit be applied by tunnelling it under the skin from the pump to the axillary artery. Where implanted, the pump is preferably powered by an implantable power source, such as for example a battery, that may be regenerated externally by an RF induction system or be replaced periodically, and/or a self-generating power source that, for example, draws energy from the human body (e.g., muscles, chemicals, heat).
The present invention also comprises a method for supplementing the circulation of blood in the patient and potentially reducing the workload on the heart of a patient without connecting any component to the patient""s heart. The inventive method comprises the steps of implanting a pump configured to generate blood flow at volumetric rates that are on average subcardiac, wherein the pump has an inflow and outflow conduit attached thereto; connecting a distal end of the inflow conduit to a first peripheral blood vessel with a minimally-invasive surgical procedure to permit the flow of blood to the pump from the first peripheral blood vessel of the patient; implanting the inflow conduit subcutaneously; connecting a distal end of the outflow conduit to a second peripheral blood vessel with a minimally-invasive surgical procedure to permit the flow of blood away from the pump to the second peripheral blood vessel of the patient; and operating said pump to perfuse blood through the patient""s circulatory system. Where the second peripheral blood vessel is the axillary artery, the step of connecting the distal end of the outflow conduit is performed in such a manner that a sufficient flow of blood is directed toward the hand to avoid limb ischemia while ensuring that sufficient flow is directed toward the aorta without damaging the endothelial lining of the second peripheral blood vessel. The same concerns for avoiding limb ischemia and damage to the endothelial lining would apply, however, regardless of the selection of second peripheral blood vessel.
In one specific application, the pump is capable of synchronous control wherein the step of operating the pump includes the steps of beginning discharge of blood out of the pump during isovolumic contraction and discontinuing discharge of blood when the aortic valve closes following systole. Depending upon the patient and the specific arrangement of the present system, this specific method results in reduced after-load and or pre-load on the heart while also supplementing circulation. For example, in one embodiment, the first and second blood vessels are the femoral and axillary arteries, respectively.
In an alternative method of applying the present invention, the pump is not implanted and the inflow and outflow conduits are connected to the first and second blood vessels percutaneously, using a readily-removable connector, such as a cannula, to connect the distal ends of each conduit to the blood vessels.
An important advantage of the present invention is that it utilizes the benefits of an IABP, without the requirement of extracorporeal equipment or the need to have a balloon or similar implement partially obstructing a blood vessel. In addition to the benefits of an IABP, it also offers the benefit of reducing the preload on the heart. The present invention thus offers simplicity and long-term use.